1. Field of the Invention
The present invention relates to devices for supplying high frequency current and voltage to power gas discharge light sources and more particularly to a novel switching inverter employed whenever direct current and voltage requirements are to be converted to alternating current and voltage at a desired frequency.
2. Brief Description of the Prior Art
Gas discharge light sources such as fluorescent lights especially those using high intensity mercury vapor, high pressure sodium, and low pressure mercury, have a negative impedance characteristic and must be operated with a ballasting device that controls the current through the source regardless of impedance changes.
It has been shown that if this current is high frequency in nature, a higher efficiency will be produced in generating light. The simplest circuit is a self-driven, high frequency inverter where current from the inverter transformer is fed back to drive switching transistors.
Simple, push-pull transistor inverters have been known for some time. The simplest form of a normal transistorized inverter uses a secondary winding arrangement on the driven transformer to supply the base drive to the transistors, either with a center tap winding or two separate windings, one for each transistor. This circuit performs well, but has some very basic inefficiencies. These inefficiencies have been tolerated due to the simplicity of design and low cost of energy. However, with the development of energy conservation, means to generate AC from DC at high efficiencies are now required. In the past, this normally has involved development of a driven inverter in which a separate circuit switches the transistors that then drive the power to the output transformer. In the self-driven circuit the output transformer must saturate before switching can occur. At saturation the OFF transistor is then turned to an ON state turning the On transistors to an OFF state. This means that both transistors are ON for a short period of time. However, due to what is called "Storage Time," the ON transistor remains in the ON state or condition until all of the charges have been moved from the junctions. This means that the time when both transistors are ON will be extended by this time, providing a temporary short circuit. High amounts of energy are consumed during this time and transient pulses that must be suppressed with energy dissipated elements are produced. The best efficiency that may be hoped from this type of circuit is around 70%, at very best 80%.
The circuit continues to have wide application and use because of its very low cost and the minimum number of parts producing high reliability. Prior electronic ballasts for fluorescent and other forms of gas discharged lighting, where high frequency current is needed, have used this circuit with a modest improvement in light operation. The typical output of this type of inverter is a square wave which has radio frequency radiation problems which reduce the effective efficiency in driving the lamp.
Recent attention on energy conservation has made it desirable to adjust the intensity of the lamp, either through local or remotely controlled dimming signals. Such circuits have not been developed to accomplish this and the small amount of prior art for dimming gas discharged tubes is exceptionally expensive, works over a very narrow range, and has been susceptible to flicker. Another prior art circuit worth considering is Letters U.S. Pat. No. 4,277,726.
Therefore, a long standing need has existed to provide a means of developing a self-driven switching inverter without the necessity to drive the switching transformer into saturation so that power losses caused by transformer core saturation are reduced as well as storage time switching delays in the switching transistors.